The present invention generally relates to electrochemical cells, i.e., batteries, and more particularly, to a closure assembly for closing the open end of a battery container and having a pressure relief mechanism for venting when exposed to excessive pressure.
Conventional alkaline electrochemical cells generally include a steel cylindrical can having a positive electrode, referred to as the cathode, which comprises manganese dioxide as the active material. The electrochemical cell also includes a negative electrode, referred to as the anode, which comprises zinc powder as the active material. In bobbin-type cell constructions, the cathode is typically formed against the interior surface of the steel can, while the anode is generally centrally disposed in the can. Alternately, in jelly-roll cells, the anode and cathode are spirally wound. A separator is located between the anode and the cathode, and an alkaline electrolyte solution simultaneously contacts the anode, the cathode, and the separator. A conductive current collector is commonly inserted into the anode active material, and a seal assembly, which includes a seal member, provides closure to the open end of the steel can to seal the active electrochemical materials in the sealed volume of the can.
Cylindrical alkaline cells are commonly sealed closed by placing a preassembled collector assembly in the open end of the steel can. The collector assembly typically includes a collector nail, an annular nylon seal, and an inner metal cover for supporting the nylon seal. The can typically has a taper or bead at its open end which serves to support the collector assembly in a desired orientation prior to securing it in place. After the collector assembly has been inserted, an outer metal cover is placed over the collector assembly and is secured in place by radially squeezing the can against the collector assembly and crimping the edge of the can over the peripheral lip of the collector assembly to secure the outer cover and collector assembly within the open end of the can. However, electrochemical cells commonly employ electrochemically active materials, such as zinc, which generate hydrogen gas during storage and sometimes during or following service use. When the battery can is sealed closed, excessive build-up of high pressure gases within the sealed can may cause damage to the cell and/or the device in which the cell is employed. xe2x80x9cVery high pressure gasses are gasses at or above the predetermined pressure which may cause damage to the cell or device in which it is usedxe2x80x9d.
One approach to avoiding a potentially excessive build-up of pressure in a cell has been to employ a resealable valve system that periodically releases excessive gas pressure from within the active cell volume. However, the continued periodic release of gas pressure may, in some situations, permit the release of electrolyte solution containing salts and other particulate matter, which may foul the resealable valve, and generally requires additional costly components that are typically quite voluminous. Other more widely acceptable approaches to venting excessive pressure have included the use of a vent formed in the annular nylon seal, which is intended to rupture upon experiencing an excessive pressure build-up in the cell. For example, U.S. Pat. No. 5,667,912 discloses a current collector assembly having a low profile seal with a thinned portion which is intended to shear when the internal pressure exceeds a predetermined pressure.
The conventional battery seals equipped with vents typically employ a circular thinned region formed in the annular nylon seal. However, the amount of space occupied by the seal and its corresponding current collector, inner metal cover, and outer metal cover, can be significant. The greater the space occupied by the seal and collector assembly, the less space that there is available within the cell for the electrochemically active materials. Consequently, a reduction in the amount of electrochemically active materials that may be provided within the cell results in a shorter service life for the cell. It is therefore desirable to maximize the internal volume within an electrochemical cell that is available for the electrochemically active components.
The present invention improves the protective safeguards of an electrochemical cell with an enhanced pressure relief mechanism that allows for use of a low profile seal assembly for sealing the open end of the cell container. To achieve this and other advantages, and in accordance with the purpose of the invention as embodied and described herein, the present invention provides for an electrochemical cell having a container with a bottom end, and an open top end, and positive and negative electrodes disposed in the container. The cell further includes an inner metal cover inserted in the open end of the container for closing the open end of the container. The inner metal cover has a pressure relief mechanism for providing pressure relief to vent high pressure gases upon reaching a predetermined pressure. In addition, an outer metal cover is positioned over the inner metal cover to form an electrical contact terminal and to substantially cover the pressure relief mechanism. The pressure relief mechanism advantageously provides for high pressure gas venting while minimizing the amount of volume consumed by the venting components and thereby allowing for a greater amount of electrochemically active materials in the container.